The present invention relates to a transfer sheet comprising a polycarbonate-based resin for printing media of electrophotographic copying machines or printers.
The transfer sheet has the function of transferring the image formed on a photosensitive drum and as occasion demands, the functions of transferring and releasing a copying paper, and usually has a drum- or belt-like shape. This transfer sheet is required to meet various and high-level property requirements such as excellent electrical and mechanical properties and high flame retardancy. Regarding the electrical properties of the transfer sheet, the charging and discharging characteristics are of much account for attaching of the copying paper on the transfer sheet, smooth carriage of the copying paper and transfer of the toner.
In use with certain kinds of copying machine, the transfer sheet is required to have excellent creep properties and fatigue fracture resistance, since the transfer sheet is mechanically deformed when releasing the copying paper. In use with another type of copying machine, the transfer sheet is required to have durability against repetitive bending since the transfer sheet is turned in a belt-like state when passed between the rolls. Such mechanical durability needs to be high enough to stand several tens of thousand times of copying.
The transfer sheet must be replaced with new one when the copy image quality has dropped due to fatigue deformation of the sheet. It is preferable that the transfer sheet be transparent because when the sheet is changed, the new one can be readily set at the accurate position and the workability is bettered. Thus, the transfer sheet is required to have a parallel ray transmittance of not less than 60%, preferably not less than 80%. Also, since the transfer sheet is a part of an electromechanical part, its flame retardancy is made much account of and the transfer sheet is required to meet the requirements of not less than V-2 level of UL (Underwriters Laboratories Inc.) Standards No. 94.
Further, there is a conflict that when it is tried to satisfy one property requirements, it is found difficult to meet other requirements. For example, when it is attempted to satisfy the requirement for flame retardancy, it is become to deteriorate the mechanical durability (fatigue strength) and the transparency. Also, when reducing a sheet thickness, the transparency becomes favorable, but the flame retardancy becomes deteriorated.
Japanese Patent Publication (KOKOKU) No. 56-25953 suggests that compositions comprising brominated polycarbonate oligomers and high-molecular weight halogenated aromatic polycarbonates are flame-retardant. Also, Japanese Patent Publication (KOKOKU) No. 55-4338 describes that the content of a brominated polycarbonate oligomer necessary for making a polycarbonate flame-retardant so as to satisfy V-1 level of UL Standards No. 94 is not less than 3% by mass.
These related arts, however, merely disclose techniques for improving flame retardancy. No suggestion nor motivation is made of applicability of the disclosed compositions to the transfer sheets, and there is no description concerning compositions of meeting the required levels of mechanical durability, transparency and flame retardancy at the same time.
Also, since brominated polycarbonate sheets are lower in fatigue strength and more liable to stress cracking than non-brominated polycarbonate sheets, mere disclosure concerning the resin compositions comprising brominated polycarbonates and brominated polycarbonate oligomers, gives no substantial hint at realizing a transfer sheet having an excellent mechanical durability (fatigue strength), a high transparency and an excellent flame retardancy.
There is a strong demand to provide a transfer sheet having excellent mechanical durability, transparency and flame retardancy, more specifically, a high fatigue fracture resistance that enables transfer of tens of thousand times of printing media, a high transparency that allows easy exchange of transfer sheet, and a flame retardancy of the not less than V-2 level of UL Standards No. 94.
As a result of the present inventors intensive studies for satisfying the above request, it has been found that by extrusion-molding a resin or a composition comprising not less than 75% by mass of a polycarbonate-based resin, the obtained sheet having characteristics that the ratio of infrared absorbance at the wave number of 155 mm.sup.-1 to that at 160 mm.sup.-1 is 0.3 to 0.6, that the melt viscosity at the shear rate of 200/sec at 280.degree. C. is not less than 2,000 Pa.multidot.s and that the thickness is 130 to 250 .mu.m, possesses not less than 4.times.10.sup.4 times of roll pass in the mechanical durability test described below, shows a parallel ray transmittance of not less than 60%, can meet the flame retardancy requirements of not less than V-2 level of UL Standards No. 94, and such sheet is useful as a transfer sheet. The present invention has been attained on the basis of this finding.